U.S. patent number 7,863,405 [Application Number 11/018,358] was granted by the patent office on 2011-01-04 for removal of residual acetaldehyde from polyester polymer particles.
This patent grant is currently assigned to Eastman Chemical Company. Invention is credited to Rodney Scott Armentrout, Frederick Leslie Colhoun, Bruce Roger DeBruin, Michael Paul Ekart.
United States Patent |
7,863,405 |
Armentrout , et al. |
January 4, 2011 |
Removal of residual acetaldehyde from polyester polymer
particles
Abstract
In one embodiment, there is provided a process comprising
introducing polyester polymer particles containing residual
acetaldehyde into a vessel at a temperature within a range of
130.degree. C. to 195.degree. C. to form a bed of particles within
the vessel, flowing a gas through at least a portion of the
particle bed, and withdrawing finished particles from the vessel
having a reduced amount of residual acetaldehyde. In this process,
it is not necessary to introduce a hot flow of gas at high flow
rates otherwise required to heat up cool particles to a temperature
sufficient to strip acetaldehyde. Rather, this process provides a
benefit in that, if desired, gas introduced into the vessel at low
flow rates and low temperatures can nevertheless effectively strip
acetaldehyde in a reasonable time because the hot particles quickly
heat a the gas to the particle temperature.
Inventors: |
Armentrout; Rodney Scott
(Kingsport, TN), Colhoun; Frederick Leslie (Kingsport,
TN), Ekart; Michael Paul (Kingsport, TN), DeBruin; Bruce
Roger (Lexington, SC) |
Assignee: |
Eastman Chemical Company
(Kingsport, TN)
|
Family
ID: |
35944287 |
Appl.
No.: |
11/018,358 |
Filed: |
December 21, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060047103 A1 |
Mar 2, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60606660 |
Sep 2, 2004 |
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Current U.S.
Class: |
528/272; 528/298;
528/491; 528/300; 528/308.8; 528/302; 528/308; 528/503;
528/492 |
Current CPC
Class: |
B01J
8/1836 (20130101); C08G 63/88 (20130101); B01J
8/1818 (20130101); B29B 2009/165 (20130101); B29B
9/065 (20130101) |
Current International
Class: |
C08G
63/02 (20060101) |
Field of
Search: |
;528/272,298,300,302,308,308.6,491,492,503 ;428/402 |
References Cited
[Referenced By]
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Other References
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Search Report for Taiwan Application No. 094130024 completed on
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European Search Report from PCT Application 05792582.8 - 2102
(PCT/US2005030531) dated Mar. 6, 2008. cited by other.
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Primary Examiner: Zemel; Irina S
Attorney, Agent or Firm: Carmen; Dennis V. Graves, Jr.;
Bernard J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application
No. 60/606,660, filed Sep. 2, 2004, the entirety of which is hereby
incorporated by reference.
Claims
What we claim is:
1. A process comprising introducing polyester polymer particles at
a temperature within a range of 130.degree. C. to 195.degree. C.
and containing residual acetaldehyde into a vessel to form a bed of
particles within the vessel, flowing a gas through at least a
portion of the particle bed at a gas flow rate not exceeding 0.15
standard cubic feet per minute (SCFM) per pound of particles per
hour, and withdrawing finished particles from the vessel having a
reduced amount of residual acetaldehyde and an It.V. of at least
0.70 dL/p.
2. The process of claim 1, wherein the particles introduced into
the vessel have an It.V. of at least 0.70 dL/g, and contain an
amount of residual acetaldehyde in excess of 10 ppm.
3. The process of claim 1, wherein the particles introduced into
the vessel comprise polyester polymers polymerized in a melt phase
to an It.V. of at least 0.72 dL/g.
4. The process of claim 3, wherein the particles have a degree of
crystallinity of at least 25% before being exposed to the flow of
gas.
5. The process of claim 1, wherein the finished particles are
introduced into a dryer to form dried particles, wherein the
finished particles dried in the dryer have not been solid state
polymerized.
6. The process of claim 1, wherein the finished particles are
loaded into a shipping container and have not been solid state
polymerized prior to loading into the container.
7. The process of claim 1, wherein the finished particles have a
residual amount of acetaldehyde of less than 5 ppm.
8. The process of claim 1, wherein the particles introduced into
the vessel comprise polyester polymers polymerized in a melt phase
to an It.V. of at least 0.72 dL/g, the particles have a degree of
crystallinity of at least 25% prior to introducing the particles
into the zone, the particles are continuously fed into the zone
without first solid state polymerizing the particles, and the
finished particles have a residual acetaldehyde amount of 7 ppm or
less.
9. The process of claim 1, wherein the gas has a nitrogen content
of less than 85% by volume.
10. The process of claim 1, wherein the gas is introduced at a
temperature of 70.degree. C. or less.
11. The process of claim 10, wherein the gas is introduced at a
temperature of 50.degree. C. or less.
12. The process of claim 1, wherein the process is continuous.
13. The process of claim 12, wherein the vessel has a particle
inlet, a particle outlet, a gas inlet, a gas outlet, and a particle
bed within the vessel, and the gas is introduced into the vessel
through the gas inlet and exits through the gas outlet after
flowing through at least a portion of the particle bed, and the
particles are introduced into the particle inlet and finished
particles are discharged from the vessel through the particle
outlet.
14. The process of claim 13, wherein the gas inlet and the finished
particle outlet are located below the gas outlet and the particle
inlet, the gas is introduced into the particle bed at any point
below 1/2 of the particle bed, the particles introduced into the
particle inlet form a bed and move by gravity to form a flow in a
direction towards the bottom of the vessel while the gas flows
countercurrent to the direction of the particle flow.
15. The process of claim 13, wherein the vessel has an aspect ratio
L/D of at least 4.
16. The process of claim 13, wherein the pressure within the vessel
measured at the gas inlet/vessel junction ranges from 0 psig to 10
psig.
17. The process of claim 1, wherein the process is continuous, the
particles are fed to the vessel at a feed rate, and for each pound
of particles fed per hour, the flow rate of the introduced gas is
at least 0.005 SCFM.
18. The process of claim 17, wherein the flow rate is not greater
than 0.10 SCFM.
19. The process of claim 18, wherein the flow rate is no greater
than 0.05 SCFM.
20. The process of claim 19, wherein the residence time of the
particles in the stripping zone ranges from 10 hours to 36
hours.
21. The process of claim 1, wherein the gas is nitrogen.
22. The process of claim 21, wherein the It.V. of the particles
introduced into the vessel comprise polyester polymers polymerized
in a melt phase to an It.V. of at least 0.72 dL/g.
23. The process of claim 22, wherein the finished particles have 5
ppm or less acetaldehyde without solid state polymerizing the
polymer.
24. The process of claim 1, wherein the residual acetaldehyde
content of the particles fed to the vessel is greater than 10 ppm,
and the amount is reduced to 10 ppm or less in the finished
particles.
25. The process of claim 1, wherein the residual acetaldehyde
content of the particles fed to the vessel is greater than 20 ppm,
and the amount is reduced to 5 ppm or less in the finished
particles.
26. The process of claim 1, wherein the polyester polymer particles
are fed to the vessel through a particle inlet, and the It.V.
differential, defined as finished particle It.V.-feed particle
It.V., is less than +0.025 dL/g.
27. The process of claim 26, wherein the It.V. differential is
+0.020 dL/g or less.
28. The process of claim 27, wherein the It.V. differential is
+0.015 dL/g or less.
29. The process of claim 1, wherein the polyester polymer particles
are fed to the vessel through a particle inlet, and the L* color
differential defined as (finished particle L* color-feed particle
L* color) is less than 5.
30. The process of claim 29, wherein the L* color differential is 3
or less.
31. The process of claim 1, wherein the polyester polymer particles
are fed to a vessel through a particle inlet, and the b* color
value of the finished particles is less than the b* color value of
the particles fed to the vessel, or is greater than the b* color
value of the particles fed to the vessel by no more than 1.0, or
remains unchanged.
32. The process of claim 31, wherein the b* remains unchanged or is
less than the b* color value of the particles fed to the
vessel.
33. The process of claim 1, wherein the particles are pellets.
34. The process of claim 1, wherein the polyester polymer
comprises: (a) a carboxylic acid component comprising at least 80
mole % of the residues of terephthalic acid, derivates of
terephthalic acid, naphthalene-2,6-dicarboxylic acid, derivatives
of naphthalene-2,6-dicarboxylic acid, or mixtures thereof, and (b)
a hydroxyl component comprising at least 60 mole % of the residues
of ethylene glycol or propane diol, based on 100 mole percent of
carboxylic acid component residues and 100 mole percent of hydroxyl
component residues in the polyester polymer.
35. The process of claim 1, wherein the acid component comprises
the residues or terephthalic acid in an amount of at least 92 mole
% and ethylene glycol in an amount of at least 92 mole %.
36. A process comprising: (a) crystallizing polyester polymer
particles to produce a hot stream of crystallized polyester polymer
particles having an average degree of crystallinity of at least
25%, residual acetaldehyde, and having a particle temperature in
excess of 90.degree. C., (b) continuously feeding into a vessel the
hot stream of crystallized polyester polymer particles at a
particle temperature of at least 130.degree. C. before the
temperature of the hot stream drops below 50.degree. C. between the
production of said crystallized polyester polymer particles and the
vessel, and (c) feeding a flow of gas into the vessel and through
at least a portion of the stream of particles at a gas flow rate
not exceeding 0.15 SCFM per pound of particles per hour to form a
stream of finished polyester polymer particles having a reduced
amount of residual acetaldehyde relative to the amount of residual
acetaldehyde prior to entry into the vessel and having an It.V. of
at least 0.70 dL/g.
37. The process of claim 36, wherein the temperature of the hot
stream is in excess of 130.degree. C.
38. The process of claim 37, wherein the hot stream is introduced
into the vessel before the temperature of the stream drops below
90.degree. C.
39. The process of claim 38, wherein the average degree of
crystallinity is at least 30%.
40. The process of claim 1, wherein the polyester polymer particles
are introduced into the vessel continuously.
41. The process of claim 26, wherein the It.V. differential is
-0.02 dL/g or more.
42. The process of claim 27, wherein the It.V. differential is
-0.02 dL/g or more.
43. A process comprising introducing polyester polymer particles at
a temperature within a range of 130.degree. C. to 195.degree. C.
and containing residual acetaldehyde into a vessel to form a bed of
particles within the vessel, flowing a gas through at least a
portion of the particle bed, and withdrawing finished particles
from the vessel having a reduced amount of residual acetaldehyde
and an It.V. of at least 0.70 dL/g, wherein said gas is introduced
into said vessel at a temperature of 70.degree. C. or less.
44. The process of claim 43, wherein said gas is introduced into
the vessel at a temperature of 60.degree. C. or less.
45. The process of claim 44, wherein said gas is introduced into
the vessel at a temperature of 50.degree. C. or less.
46. The process of claim 45, wherein said gas is introduced into
the vessel at a temperature of 40.degree. C. or less.
47. The process of claim 46, wherein said gas is introduced into
the vessel at about ambient air temperature.
48. The process of any one of claims 43-47, wherein the gas flow
rate is less than 0.05 SCFM per pound of particles per hour.
49. The process of any one of claims 43-47, wherein the finished
particles are introduced into a dryer to form dry particles fed to
a melt processing zone.
50. The process of any one of claims 43-47, wherein the finished
particles are fed to a melt processing zone, and the finished
particles have not been solid state polymerized.
51. The process of claim 43, wherein the finished particles are
loaded into a shipping container and have not been solid state
polymerized prior to loading into the container.
52. The process of claim 51, wherein the finished particles have a
residual acetaldehyde amount of less than 5 ppm.
53. The process of claim 43, wherein the process is continuous.
54. The process of claim 43, wherein the vessel has a particle
inlet, a particle outlet, a gas inlet, a gas outlet, and a particle
bed within the vessel, wherein the gas inlet and the finished
particle outlet are located below the gas outlet and the particle
inlet, and the gas is introduced into the particle bed at any point
below one-half of the distance of the particle bed, and the
particles that are introduced into the particle inlet form a bed
and move by gravity to form a flow in a direction towards the
bottom of the vessel while the gas flows countercurrent to the
direction of the particle flow.
55. The process of claim 43, wherein the gas is ambient air.
56. The process of claim 43, wherein the finished particles have 5
ppm or less acetaldehyde without solid state polymerizing the
polymer.
57. The process of claim 43, wherein the residual acetaldehyde in
the particles fed to the vessel is greater than 20 ppm.
58. The process of claim 43, wherein the particles introduced into
the vessel (feed particles) and the finished particles have an
It.V., and the It.V. differential, defined as finished particle
It.V.-feed particle It.V. is -0.02 dL/g or more.
59. The process of claim 58, wherein the It.V. differential is
+0.105 dL/g or less.
60. The process of claim 43, wherein said gas is nitrogen gas.
61. The process of claim 1, wherein the crystallized polyester
polymer particles comprise strain crystallized particles.
Description
FIELD OF THE INVENTION
This invention relates to the removal of residual acetaldehyde from
polyester particles.
BACKGROUND OF THE INVENTION
A conventional process for the preparation of a polyethylene
terephthalate based resin (PET) is characterized as a two stage
process: a melt phase process which includes the esterification and
polycondensation reactions, and a solid state polymerization
process for increasing the molecular weight of the polymer in the
solid state rather than in the melt. In a solid state
polymerization process, PET is exposed to temperatures of
200-230.degree. C. and a constant counter-current flow of nitrogen
through the resin for a significant length of time. In such a
conventional process, the molecular weight of the resin is
increased in the melt phase up to an It.V. of about 0.55 to 0.65,
followed by pelletization, after which the pellets are
crystallized, and then solid state polymerized with an optional
annealing step after crystallization.
In the melt phase, residual acetaldehyde is formed by degradation
reactions occurring at the high temperatures experienced during the
last stages of polycondensation. In a conventional process,
attempting to further increase the molecular weight at these It.V.
levels causes a marked increase in the formation of acetaldehyde.
However, elevated temperatures in the melt phase are required to
facilitate the polycondensation molecular weight building
reactions. Accordingly, the polymer is made only to a low It.V. of
about 0.55 to 0.60 dL/g in the melt phase, followed eventually by
the further increase in the molecular weight of the polymer in the
solid state.
During solid state polymerization, the particles are exposed to a
counter-current flow of nitrogen gas to carry off ethylene glycol,
water, and/or other condensates generated during polycondensation.
The use of nitrogen also minimizes the oxidative degradation of the
PET resin at solid stating temperatures. The nitrogen gas also
helps safeguard against oxidation of antimony metal in resins
containing reduced antimony as a reheat agent. Although the solid
state polymerization provides a product with limited degradation
products, the process adds a considerable amount of cost
(conversion and capital) to the PET manufacturing process.
It would be desirable to eliminate the step of solid state
polymerization by the manufacture of a polyester polymer resin in
the melt phase having a high It.V. while minimizing the level of
residual acetaldehyde, while also providing a crystallized particle
to reduce the agglomeration of the particles in dryers feeding
extruders for the formation of articles such as preforms and
sheet.
SUMMARY OF THE INVENTION
In one embodiment, there is provided a process comprising
introducing polyester polymer particles containing residual
acetaldehyde into a vessel at a temperature within a range of
130.degree. C. to 195.degree. C. to form a bed of particles within
the vessel, flowing a gas through at least a portion of the
particle bed, and withdrawing finished particles from the vessel
having a reduced amount of residual acetaldehyde. In this process,
it is not necessary to introduce a hot flow of gas at high flow
rates otherwise required to heat up cool particles to a temperature
sufficient to strip acetaldehyde. Rather, this process provides a
benefit in that, if desired, gas introduced into the vessel at low
flow rates and low temperatures can nevertheless be effective to
strip acetaldehyde in a reasonable time because the hot particles
quickly heat the low flow of gas to the particle temperature.
In a variety of other embodiments, the polyester polymer forming
the particles is polymerized in the melt phase to an It.V. of at
least 0.72 dL/g, or the particles are partially crystallized before
being exposed to the flow of gas, or the polyester polymer
particles finished by the above method are dried in a dryer and fed
to a melt processing zone without solid state polymerizing the
particles, or the finished polyester polymer particles have a
residual level of acetaldehyde of less than 5 ppm, or the process
comprises a combination of any two or more of these features.
In yet another embodiment, there is provided a process comprising
crystallizing polyester polymer particles to produce a hot stream
of crystallized polyester polymer particles having an average
degree of crystallinity of at least 25% and having a particle
temperature in excess of 90.degree. C., continuously feeding the
hot stream of particles at a temperature of at least 130.degree. C.
into a vessel before the temperature of the hot stream drops below
50.degree. C., feeding a flow of gas into the vessel and through
the stream of particles in an amount sufficient to form a stream of
finished polyester polymer particles having a reduced level of
residual acetaldehyde relative to the level residual acetaldehyde
prior to entry into the vessel. In this embodiment, heat energy
imparted to particles during crystallization is harnessed as the
heat energy transferred to the gas in the stripping vessel needed
to reduce the level of residual acetaldehyde on or in the
particles.
There is also provided a process comprising continuously feeding a
stream of polyester polymer particles having a residual
acetaldehyde level into a vessel, allowing the particles to form a
bed and flow by gravity to the bottom of the vessel, continuously
withdrawing finished particles from the vessel having a residual
acetaldehyde level which is less than the residual acetaldehyde
level of the stream of particles fed to the vessel and in no event
greater than 10 ppm, continuously introducing a flow of gas into
the vessel, and passing the flow of gas through the particles
within the vessel, wherein the particles introduced into the vessel
have an It.V. of at least 0.72 dL/g obtained without polymerization
in the solid state. In this embodiment, particles having high It.V.
and low levels of residual acetaldehyde are made without the need
for solid state polymerization, thereby avoiding the costly solid
state polymerization step.
In all of these embodiments, the use of costly acetaldehyde
scavengers can also be avoided if desired.
These and other features of the invention are described in further
detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an acetaldehyde stripping vessel.
FIG. 2 is a process flow diagram for crystallizing and stripping
acetaldehyde from polyester polymer particles.
FIG. 3 illustrates a lab model of a modified chromatograph column
used to conduct experiments.
DETAILED DESCRIPTION OF THE INVENTION
The present invention may be understood more readily by reference
to the following detailed description of the invention. It is to be
understood that this invention is not limited to the specific
processes and conditions described, as specific processes and/or
process conditions for processing plastic articles as such may, of
course, vary.
It must also be noted that, as used in the specification and the
appended claims, the singular forms "a", "an" and "the" include
plural referents. References to a composition containing "an"
ingredient or "a" polymer is intended to include other ingredients
or other polymers, respectively, in addition to the one named.
Ranges may be expressed herein as "within" or "between" or from one
value to another. In each case, the end points are included in the
range. Ranges expressed as being greater than or less than a value
exclude the end point(s).
By "comprising" or "containing" or "having" is meant that at least
the named compound, element, particle, or method step etc must be
present in the composition or article or method, but does not
exclude the presence of other compounds, materials, particles,
method steps, etc, even if the other such compounds, material,
particles, method steps etc. have the same function as what is
named.
Regardless of the context, the expression of a temperature means
the temperature applied to the polymer unless otherwise expressed
as the "actual" polymer temperature.
It is also to be understood that the mention of one or more method
steps does not preclude the presence of additional method steps or
intervening method steps between those steps expressly
identified.
In the first embodiment of the invention, polyester polymer
particles containing residual acetaldehyde are introduced into a
vessel at a temperature within a range of 130.degree. C. to
195.degree. C. to form a bed of particles within the vessel, a flow
of gas is allowed to pass through at least a portion of the
particle bed, and finished particles are withdrawn from the vessel
having a reduced amount of residual acetaldehyde.
In this first embodiment, a stream of polyester polymer particles
is fed into the vessel at an elevated temperature. The elevated
temperature is at least 130.degree. C., or at least 140.degree. C.,
or at least 150.degree. C., or at least 160.degree. C., and
preferably under 195.degree. C., or 190.degree. C. or less. By
feeding a stream of hot particles to the stripping vessel, costs
associated with heating a flow of gas or providing for a high gas
flow rate are avoided if desired. The hot particles provide the
heat energy transferred to the gas to provide a gas temperature
within the vessel sufficient to effectuate acetaldehyde
stripping.
The polyester polymer particles introduced into the vessel contain
a level of residual acetaldehyde. The invention reduces the amount
of acetaldehyde present in the polyester polymer particles fed to
the acetaldehyde stripping vessel. In one embodiment, the level of
residual acetaldehyde present in the particles fed to the vessel is
greater than 10 ppm, or greater than 20 ppm, or 30 ppm or more, or
40 ppm or more, and even 50 ppm or more.
Finished particles are particles treated by a flow of gas and
having a level of residual acetaldehyde which is less than the
level of residual acetaldehyde present on or in the particles fed
to the vessel. Preferably, the level of residual acetaldehyde
present on the finished particles is 10 ppm or less, or 7 ppm or
less, or 5 ppm or less, or 3 ppm or less, or 2 ppm or less, or 1.5
ppm or less. In another embodiment, the reduction in acetaldehyde
is at least 5 ppm, or at least 10 ppm, or at least 20 ppm, or at
least 30 ppm. When a relative comparison is made, the amount of
residual acetaldehyde can be measured according to standard
techniques in the industry so long as the same test method is used.
Otherwise, the test method used to determine the residual
acetaldehyde content is ASTM F2013-00 "Determination of Residual
Acetaldehyde in Polyethylene Terephthalate Bottle Polymer Using an
Automated Static Head-Space Sampling Device and a Capillary GC with
a Flame Ionization Detector".
The polyester polymer particles are exposed to a flow of gas across
the particles in the particle bed within the vessel. The
temperature of the gas as introduced into the vessel containing the
bed of particles is desirably within a range of 0.degree. C. to
200.degree. C. At the preferred low gas flow rates described below,
the gas temperature quickly equilibrates to the particle
temperature in the bed within the vessel. For example, gas
introduced at a temperature higher than the temperature of the
particles will quickly equilibrate to the lower particle
temperature at low gas flow rates relative to the flow rate of the
particles introduced into the vessel. Likewise, gas introduced into
the vessel at a temperature lower than the temperature of the
particles will quickly equilibrate to the higher particle
temperature at low gas flow rates relative to the flow rate of the
particles introduced into the vessel. While it is possible to
introduce gas at high temperature into the vessel, it is
unnecessary and represents a waste of energy to heat the gas.
Therefore, it is preferred to introduce gas into the vessel at the
low end of the temperature spectrum. In a more preferred
embodiment, the gas is introduced into the vessel at a temperature
of 70.degree. C. or less, or 60.degree. C. or less, or 50.degree.
C. or less, or 40.degree. C. or less, and preferably at 10.degree.
C. or more, or 15.degree. C. or more, or 20.degree. C. or more, and
most preferably is introduced at about the ambient air
temperature.
Signs of oxidation and/or polycondensation reactions include an
increase in the It.V. of the particles, or a change in L*, a*,
and/or b* color, or a combination of two or more of these signs. To
prevent polycondensing or oxidizing the polyester polymer to any
significant extent, the temperature of the gas exiting the
stripping vessel is preferably 195.degree. C. or less.
The gas can be introduced into the vessel by any conventional
means, such as by a blower, fans, pumps, and the like. The gas may
flow co-current to or countercurrent to or across the flow of
particles through the vessel. The preferred flow of gas through the
bed of particles is countercurrent to the particle flow through the
bed. The gas can be introduced at any desired point on the vessel
effective to reduce the level of acetaldehyde on the particles fed
to the vessel. Preferably, the gas introduction point in to the
lower half of the bed height, and more preferably to the lower 1/4
of the bed height. The gas flows through at least a portion of the
particle bed, preferably through at least 50 volume % of the bed,
more preferably through at least 75% of the particle bed
volume.
Any gas is suitable for use in the invention, such as air, carbon
dioxide, and nitrogen. Some gases are more preferred than others
due to the ready availability and low cost. For example, the use of
air rather than nitrogen would lead to significant operating cost
improvements. It was believed that the use of nitrogen gas was
required in operations which pass a hot flow of gas through a bed
of particles, such as in a crystallizer, because nitrogen is inert
to the oxidative reactions which would otherwise occur between many
polyester polymers and ambient oxygen resulting in pellet
discoloration. However, by keeping the process temperature low such
that the gas exiting the vessel does not exceed 195.degree. C.,
particle discoloration is minimized. In one embodiment, the gas
contains less than 90 vol % nitrogen, or less than 85 vol %
nitrogen, or less than 80 vol % nitrogen. In another embodiment,
the gas contains oxygen in an amount of 17.5 vol % or more. The use
of air at ambient composition (the composition of the air at the
plant site on which the vessel is located), or air which is not
separated or purified, is preferred. Desirably, ambient air is fed
through the gas inlet. While the air can be dried if desired, it is
not necessary to dry the air since the object of the invention is
to strip acetaldehyde from the particles.
Any vessel for containing particles and allowing a feed of gas and
particles into and out of the vessel is suitable. For example,
there is provided a vessel having at least an inlet for gas, and
inlet for the polyester polymer particles, an outlet for the gas,
and an outlet for the finished particles. The vessel preferably
insulated to retain heat. The gas inlet and the finished particle
outlet is desirably located below the gas outlet and the particle
inlet, preferably with the latter being toward the top of the
vessel and the former being toward the bottom of the vessel. The
gas is desirably introduced into the bed within the vessel at about
1/2 or 1/4, of the bed height within the vessel. The particles are
preferably introduced at the top of the vessel, and move by gravity
to the bottom of the vessel, while the gas preferably flows
countercurrent to the direction of the particle flow. The particles
accumulate within the vessel to form a bed of particles, and the
particles slowly descend down the length of the vessel by gravity
to the finished particle outlet at the bottom of the vessel. The
bed height is not limited, but is preferably at a substantially
constant height in a continuous process and is at least 75% of the
height of the stripping zone containing the particles within the
vessel. The vessel preferably has an aspect ratio L/D of at least
2, or at least 4, or at least 6. While the process can be conducted
in a batch or semi batch mode in which as the particles would not
flow and the stream of gas can be passed through the bed of
particles in any direction, the process is preferably continuous in
which a stream of particles continuously flows from the particle
inlet to the finished particle as the particles are fed to the
vessel.
A suitable gas flow rate introduced into the vessel and passing
through at least a portion of the particle bed is one which is
sufficient to reduce the amount of residual acetaldehyde on the
particles introduced into the vessel. However, to obtain one of the
advantages of the invention, that is, lowering energy requirements
on the gas and reducing capital costs on the gas, the gas flow rate
at the gas inlet is low. For example, for every one (1) pound of
particles charged to the vessel per hour, suitable gas flow rates
introduced into the vessel are at least 0.0001 standard cubic feet
per minute (SCFM), or at least 0.001 SCFM, or at least 0.005 SCFM.
High flow rates are also suitable, but not necessary, and should be
kept sufficiently low to avoid unnecessary energy consumption by
the gas pumps, fans, or blowers. Moreover, it is not desired to
unduly cool the particles or dry particles, both objects which
typically require the use of high gas flow rates to achieve. The
gas flow rate in the process of the invention is preferably not any
higher than 0.15 SCFM, or not higher than 0.10 SCFM, or not higher
than 0.05 SCFM, or even not higher than 0.01 SCFM for every one (1)
pound of charged particles per hour. The optimal flow rate is
desirably set to provide the needed level of acetaldehyde removal
without unnecessary energy consumption. Moreover, by providing low
gas flow rates to the vessel, the gas is quickly heated within the
vessel by the hot particles, thereby providing a hot gas throughout
a substantial portion of the particle bed within the vessel
effective to strip residual acetaldehyde from the particles.
Since the inlet gas pressure can be substantially atmospheric or at
very low pressure, suitable devices to move the gas through the
vessel are advantageously fans or blowers, although any suitable
device for providing a motive force to a gas can be used.
If desired, the residence time of the particles can be shortened by
increasing the temperature at which stripping occurs. This
temperature is largely controlled by the temperature of the
particles introduced into the vessel. The heat transfer from the
particles rapidly heat the gas after it enters the vessel. At the
point where the gas enters the vessel, the particles undergo a
temperature change depending on the flow rate and temperature of
the gas.
An additional advantage of this process is the capability to
integrate the heat energy between different steps for producing
polyester polymer particles in that the hot gas stream exiting the
vessel can now be used to provide heat transfer to other suitable
parts of a polyester polymer plant or as a source of combustion,
such as a source of hot gas to a furnace to lower the energy
requirements of the furnace.
The overall process for making polyester polymer resin, however,
becomes much more economical if the crystallized particles
introduced into the acetaldehyde stripping zone do not have to be
heated up to temperature after crystallization. Allowing the
crystallized particles to cool after crystallization, followed by
heating the particles back up to the desired introductory
temperature for acetaldehyde stripping, wastes energy. Accordingly,
there is provided an integrated process wherein polyester polymer
particles are crystallized in a crystallization zone, discharged as
a stream of particles from the crystallization zone at particle
temperatures in excess of 90.degree. C., or in excess of
100.degree. C., or in excess of 120.degree. C., or in excess of
130.degree. C., or even in excess of 140.degree. C., and before the
stream of particles is allowed to drop to a temperature below
50.degree. C., or below 70.degree. C., or below 90.degree. C., or
below 130.degree. C., the stream of hot particles is fed to an
acetaldehyde stripping zone in which a flow of gas is introduced at
a temperature within a range of about 0.degree. C. to 250.degree.
C., and the gas is passed through the stream of polyester polymer
particles in an amount sufficient to form a stream of finished
polyester polymer particles having a reduced level of the residual
acetaldehyde.
The degree of crystallinity of the polyester polymer particles is
not particularly limited. It is preferred to employ crystallizable
polyester polymers. The process of the invention is capable of
producing high It.V. polyester polymer particles having low levels
of residual acetaldehyde ready to be shipped or fed to a dryer
feeding an injection molding machine or extruder for making an
article, such as sheet or preforms. To reduce the tendency of the
particles to stick to each other in the dryer, it is preferred to
feed the dryer with partially crystallized particles. Therefore, in
one embodiment, the polyester polymer particles fed to the
acetaldehyde stripping zone are partially crystallized, preferably
to a degree of crystallinity of at least 25%, or at least 30%, or
at least 35%, and up to about 60%. The particles can be
crystallized to a higher degree of crystallinity, but satisfactory
results in decreasing the level of particles agglomeration can be
obtained within these ranges.
The pressure within the vessel is not particularly limited. The
vessel can be maintained close to ambient conditions, with a slight
amount of pressure to force gas into the vessel. Within the vessel,
a slight pressure gradient will exist if hot particles are
introduced from the air inlet to the air outlet. A pressure
gradient also exists due to the pressure drop from friction of the
gas on the pellets. The pressure within the vessel measured at the
gas inlet close to the gas inlet/vessel junction ranges from about
0 psig to about 30 psig, preferably from about 0 psig to about 10
psig, or from about 0 psig to 5 psig.
The residence time of the particles in contact with the flow of gas
within the vessel is also not particularly limited. Suitable
residence times range from 2 hours, or from 10 hours, or from 18
hours, and up to about 48 hours, or 36 hours, or 30 hours.
The process of the invention provides the flexibility of adjusting
a number of variables to maintain a constant particle It.V. and to
mitigate discoloring the particles. The process variables include
the, particle introductory temperature, the particle residence
time, the gas flow rate, and the gas introductory temperature.
Optimal process conditions to minimize oxidation reactions,
discoloration, maintain the It.V. of the particles, and remove
acetaldehyde while keeping the production costs low are to
introduce the gas at ambient temperature, to feed particles within
a range of 150.degree. C. to 170.degree. C. into a vertical
cylindrical vessel at an air flow rate ranging from 0.002 SCFM to
0.009 SCFM per 1 lb of PET. The size of the vessel is such that the
residence time of the pellets averages about 10 to 24 hours.
The process of the invention provides an advantageous low cost
means for reducing residual acetaldehyde from a polyester polymer
having a high molecular weight and high It.V., such as at least
0.70 dL/g. The low level of acetaldehyde may also be obtained
without the need for adding an acetaldehyde scavenging compound
into the melt phase for the production of the high It.V. polyester
polymer. Thus, there are provided several additional embodiments
comprising: 1. A polyester polymer resin having an It.V. of at
least 0.70 dL/g and 5 ppm or less acetaldehyde without solid state
polymerizing the polymer; 2. A polyester polymer resin made in a
melt phase to an It.V. of at least 0.70 dL/g without adding an
acetaldehyde scavenger to the polymer during melt phase production,
the polyester polymer having an acetaldehyde content of 5 ppm or
less acetaldehyde, and preferably an acetaldehyde content of 5 ppm
or less without solid state polymerizing the polymer.
In conventional polyester production technology, the polyester
polymer is polymerized in the melt to a relatively low It.V. of 0.5
to about 0.65 dL/g partly because a further increase in It.V.
results in the build up of unacceptably high levels of
acetaldehyde. As a result, the molecular weight of the polymer is
further advanced in the solid state rather than in a melt to avoid
further increased, and to actually decrease, the levels of residual
acetaldehyde. With the process of the invention, however, to solid
state polymerization process may be avoided altogether while
obtaining a particle with low residual acetaldehyde. Thus, there is
also provided another embodiment where a stream of polyester
polymer particles having a residual acetaldehyde level are fed
continuously into a vessel, allowed to form a bed and flow by
gravity to the bottom of the vessel, continuously withdrawn from
the vessel as finished particles having a residual acetaldehyde
level which is less than the residual acetaldehyde level of the
stream of particles fed to the vessel and in no event greater than
10 ppm, continuously introducing a flow of gas into the vessel, and
passing the flow of gas through the particles within the vessel,
wherein the particles introduced into the vessel have an It.V. of
at least 0.72 dL/g obtained without polymerization in the solid
state.
The finished particles are directly or indirectly packaged into
shipping containers, which are then shipped to customers or
distributors. It is preferred to subject the crystallized particles
to any process embodiment described herein without solid state
polymerizing the particles at any point prior to packaging the
particles into shipping containers. With the exception of solid
state polymerization, the particles may be subjected to numerous
additional processing steps in-between any of the expressed
steps.
Shipping containers are containers used for shipping over land, sea
or air. Examples include railcars, semi-containers, Gaylord boxes,
and ship hulls.
One of the advantages of the invention is that the stripping
process is conducted at a temperature low enough where the polymer
does not polycondense and build up molecular weight. Thus, in an
embodiment of the invention, process conditions are established
such that the It.V. differential measured as the It.V. of the
finished polyester polymer and the It.V. of the polyester polymer
fed to the acetaldehyde stripping zone, is less than +0.025 dL/g,
or +0.020 dL/g or less, or +0.015 dL/g or less, or +0.010 dL/g or
less, and preferably -0.02 dL/g or more, or -0.01 dL/g or more, and
most preferably close to 0, within experimental error.
The It.V. values described throughout this description are set
forth in dL/g units as calculated from the inherent viscosity
measured at 25.degree. C. in 60/40 wt/wt phenol/tetrachloroethane.
The inherent viscosity is calculated from the measured solution
viscosity. The following equations describe such solution viscosity
measurements and subsequent calculations to Ih.V. and from Ih.V. to
It.V: .eta..sub.inh=[ln(t.sub.s/t.sub.o)]/C where
.eta..sub.inh=Inherent viscosity at 25.degree. C. at a polymer
concentration of 0.50 g/100 mL of 60% phenol and 40%
1,1,2,2-tetrachloroethane ln=Natural logarithm t.sub.s=Sample flow
time through a capillary tube t.sub.o=Solvent-blank flow time
through a capillary tube C=Concentration of polymer in grams per
100 mL of solvent (0.50%)
The intrinsic viscosity is the limiting value at infinite dilution
of the specific viscosity of a polymer. It is defined by the
following equation:
##STR00001##
where .eta..sub.int=Intrinsic viscosity .eta..sub.r=Relative
viscosity=t.sub.s/t.sub.o .eta..sub.sp=Specific
viscosity=.eta..sub.r-1
Instrument calibration involves replicate testing of a standard
reference material and then applying appropriate mathematical
equations to produce the "accepted" I.V. values. Calibration
Factor=Accepted IV of Reference Material/Average of Replicate
Determinations Corrected IhV=Calculated IhV.times.Calibration
Factor
The intrinsic viscosity (ItV or .eta..sub.int) may be estimated
using the Billmeyer equation as follows:
.eta..sub.int=0.5[e.sup.0.5.times.Corrected
IhV-1]+(0.75.times.Corrected IhV)
There is also provided an embodiment where the process conditions
are established such that the L* color value differential measured
as (L* finished polyester polymer-L* of the particle feed) is 5 or
less, or 3 or less, or 2 or less, and desirably greater than -3, or
greater than -2, or greater than -1. Preferred L* value
differentials are close to 0. While positive changes where the L*
is actually increased in the finished polymer are acceptable and
even desirable, consideration should be taken into account as to
the reason why the L* is increased. In some cases, L* can increase
due to the oxidation of a metal, which may or may not be a
significant consideration depending upon the function of the metal.
If the metal is present as a reheat additive, its function as a
reheat additive will diminish if oxidized even though the L* color
brightness increases. In this case, the amount of metal present can
be increased proportionately to allow for the presence of
sufficient elemental metal to act as a reheat additive, but in many
cases, the amount of metal remaining after its oxidation to
function as a reheat agent is a balance against the additional
brightness obtained as indicated by the increase in L*. The
particular end use application and cost will control the degree of
increase in L* and reduction in reheat which can be tolerated.
However, if the function of the metal is already served or not
impacted by an oxidation reaction, then an increase in L* to any
degree may actually be desired.
Another advantage of the invention is that the stripping process is
conducted under conditions to prevent the polymer from exhibiting a
significant change in color in the direction toward more
yellowness. Accordingly, there is provided another embodiment in
which process conditions are established such that the b* color
value of the finished polyester polymer is less than the b* color
value of the polyester polymer fed to the acetaldehyde stripping
zone, or is unchanged, or is greater than by not more than 1.0, but
is preferably unchanged or less. For example, a finished particle
b* color value of -2.1 is less than a feed particle b* color value
of -1.5. Likewise, a finished b* color value of +2.0 is less than a
feed particle b* color value of +2.7. b* color value shifts in the
direction toward the blue end of the b* color spectrum is
desirable. In this way, the process conditions do not add a
substantially greater yellow hue to the particles.
The measurements of L*, a*, and b* color values are conducted
according to the following methods. The instrument used for
measuring color should have the capabilities of a HunterLab
UltraScan XE, model U3350, using the CIELab Scale(L*, a*, b*), D65
(ASTM) illuminant, 100 observer, integrating sphere geometry.
Particles are measured in RSIN reflection, specular component
included mode according to ASTM D6290, "Standard Test Method for
Color Determination of Plastic Pellets". Plastic pellets are placed
in a 33-mm path length optical glass cell, available from
HunterLab, and allowed to settle by vibrating the sample cell using
a laboratory Mini-Vortexer (VWR International, West Chester, Pa.).
The instrument for measuring color is set up under ASTM E1164
"Standard Practice for Obtaining Spectrophotometric Data for
Object-Color Evaluation." Color is determined on a sample by using
its absolute value-the value determined by the instrument.
The measurements of % crystallinity are obtained from differential
scanning calorimetry according to the following equation: %
crystallinity=[.DELTA.Hm/.DELTA.Hm.degree.]100% where .DELTA.Hm is
the heat of melting of the polymer determined by integrating the
area under the curve (Joule/gram) of the melting transition(s)
observed during the first scan of 25.degree. C. to 300.degree. C.
at 20.degree. C. per minute in a Perkin Elmer differential scanning
calorimeter and .DELTA.Hm.degree. is a reference value of 140.1 J/g
and represents the heat of melting if the polyethylene
terephthalate is 100% crystalline.
The shape of the polyester polymer particles is not limited, and
can include regular or irregular shaped discrete particles without
limitation on their dimensions, including, stars, spheres,
spheroids, globoids, cylindrically shaped pellets, conventional
pellets, pastilles, and any other shape, but particles are
distinguished from a sheet, film, preforms, strands or fibers.
The number average weight (not to be confused with the number
average molecular weight) of the particles is not particularly
limited. Desirably, the particles have a number average weight of
at least 0.10 g per 100 particles, more preferably greater than 1.0
g per 100 particles, and up to about 100 g per 100 particles.
The polyester polymer of this invention is any thermoplastic
polyester polymer. A polyester thermoplastic polymers of the
invention are distinguishable from liquid crystal polymers and
thermosetting polymers in that thermoplastic polymers have no
ordered structure while in the liquid (melt) phase, they can be
remelted and reshaped into a molded article, and liquid crystal
polymers and thermosetting polymers are unsuitable for the intended
applications such as packaging or stretching in a mold to make a
container.
The polyester polymer desirably contains alkylene terephthalate or
alkylene naphthalate units in the polymer chain. More preferred are
polyester polymers which comprise: (a) a carboxylic acid component
comprising at least 80 mole % of the residues of terephthalic acid,
derivates of terephthalic acid, naphthalene-2,6-dicarboxylic acid,
derivatives of naphthalene-2,6-dicarboxylic acid, or mixtures
thereof, and (b) a hydroxyl component comprising at least 60 mole
%, or at least 80 mole %, of the residues of ethylene glycol or
propane diol, based on 100 mole percent of carboxylic acid
component residues and 100 mole percent of hydroxyl component
residues in the polyester polymer.
Typically, polyesters such as polyethylene terephthalate are made
by reacting a diol such as ethylene glycol with a dicarboxylic acid
as the free acid or its dimethyl ester to produce an ester monomer
and/or oligomers, which are then polycondensed to produce the
polyester. More than one compound containing carboxylic acid
group(s) or derivative(s) thereof can be reacted during the
process. All the compounds containing carboxylic acid group(s) or
derivative(s) thereof that are in the product comprise the
"carboxylic acid component." The mole % of all the compounds
containing carboxylic acid group(s) or derivative(s) thereof that
are in the product add up to 100. The "residues" of compound(s)
containing carboxylic acid group(s) or derivative(s) thereof that
are in the product refers to the portion of said compound(s) which
remains in the oligomer and/or polymer chain after the condensation
reaction with a compound(s) containing hydroxyl group(s).
More than one compound containing hydroxyl group(s) or derivatives
thereof can become part of the polyester polymer product(s). All
the compounds containing hydroxyl group(s) or derivatives thereof
that become part of said product(s) comprise the hydroxyl
component. The mole % of all the compounds containing hydroxyl
group(s) or derivatives thereof that become part of said product(s)
add up to 100. The residues of hydroxyl functional compound(s) or
derivatives thereof that become part of said product refers to the
portion of said compound(s) which remains in said product after
said compound(s) is condensed with a compound(s) containing
carboxylic acid group(s) or derivative(s) thereof and further
polycondensed with polyester polymer chains of varying length.
The mole % of the hydroxyl residues and carboxylic acid residues in
the product(s) can be determined by proton NMR.
In another embodiment, the polyester polymer comprises: (a) a
carboxylic acid component comprising at least 90 mole %, or at
least 92 mole %, or at least 96 mole % of the residues of
terephthalic acid, derivates of terephthalic acid,
naphthalene-2,6-dicarboxylic acid, derivatives of
naphthalene-2,6-dicarboxylic acid, or mixtures thereof, and (b) a
hydroxyl component comprising at least 90 mole %, or at least 92
mole %, or at least 96 mole % of the residues of ethylene glycol,
based on 100 mole percent of the carboxylic acid component residues
and 100 mole percent of the hydroxyl component residues in the
polyester polymer.
The reaction of the carboxylic acid component with the hydroxyl
component during the preparation of the polyester polymer is not
restricted to the stated mole percentages since one may utilize a
large excess of the hydroxyl component if desired, e.g. on the
order of up to 200 mole % relative to the 100 mole % of carboxylic
acid component used. The polyester polymer made by the reaction
will, however, contain the stated amounts of aromatic dicarboxylic
acid residues and ethylene glycol residues.
Derivates of terephthalic acid and naphthalane dicarboxylic acid
include C.sub.1-C.sub.4 dialkylterephthalates and C.sub.1-C.sub.4
dialkylnaphthalates, such as dimethylterephthalate and
dimethylnaphthalate.
In addition to a diacid component of terephthalic acid, derivates
of terephthalic acid, naphthalene-2,6-dicarboxylic acid,
derivatives of naphthalene-2,6-dicarboxylic acid, or mixtures
thereof, the carboxylic acid component(s) of the present polyester
may include one or more additional modifier carboxylic acid
compounds. Such additional modifier carboxylic acid compounds
include mono-carboxylic acid compounds, dicarboxylic acid
compounds, and compounds with a higher number of carboxylic acid
groups. Examples include aromatic dicarboxylic acids preferably
having 8 to 14 carbon atoms, aliphatic dicarboxylic acids
preferably having 4 to 12 carbon atoms, or cycloaliphatic
dicarboxylic acids preferably having 8 to 12 carbon atoms. More
specific examples of modifier dicarboxylic acids useful as an acid
component(s) are phthalic acid, isophthalic acid,
naphthalene-2,6-dicarboxylic acid, cyclohexanedicarboxylic acid,
cyclohexanediacetic acid, diphenyl-4,4'-dicarboxylic acid, succinic
acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, and
the like, with isophthalic acid, naphthalene-2,6-dicarboxylic acid,
and cyclohexanedicarboxylic acid being most preferable. It should
be understood that use of the corresponding acid anhydrides,
esters, and acid chlorides of these acids is included in the term
"carboxylic acid". It is also possible for tricarboxyl compounds
and compounds with a higher number of carboxylic acid groups to
modify the polyester.
In addition to a hydroxyl component comprising ethylene glycol, the
hydroxyl component of the present polyester may include additional
modifier mono-ols, diols, or compounds with a higher number of
hydroxyl groups. Examples of modifier hydroxyl compounds include
cycloaliphatic diols preferably having 6 to 20 carbon atoms and/or
aliphatic diols preferably having 3 to 20 carbon atoms. More
specific examples of such diols include diethylene glycol;
triethylene glycol; 1,4-cyclohexanedimethanol; propane-1,3-diol;
butane-1,4-diol; pentane-1,5-diol; hexane-1,6-diol;
3-methylpentanediol-(2,4); 2-methylpentanediol-(1,4);
2,2,4-trimethylpentane-diol-(1,3); 2,5-ethylhexanediol-(1,3);
2,2-diethyl propane-diol-(1,3); hexanediol-(1,3);
1,4-di-(hydroxyethoxy)-benzene;
2,2-bis-(4-hydroxycyclohexyl)-propane;
2,4-dihydroxy-1,1,3,3-tetramethyl-cyclobutane;
2,2-bis-(3-hydroxyethoxyphenyl)-propane; and
2,2-bis-(4-hydroxypropoxyphenyl)-propane.
As modifiers, the polyester polymer may preferably contain such
comonomers as such as isophthalic acid, naphthalane dicarboxylic
acid, cyclohexanedimethanol, and diethylene glycol.
The polyester pellet compositions may include blends of
polyalkylene terephthalates and/or polyalkylene naphthalates along
with other thermoplastic polymers such as polycarbonate (PC) and
polyamides. It is preferred that the polyester composition should
comprise a majority of the polyester polymers, more preferably in
an amount of at least 80 wt. %, or at least 95 wt. %, and most
preferably 100 wt. %, based on the weight of all thermoplastic
polymers (excluding fillers, inorganic compounds or particles,
fibers, impact modifiers, or other polymers which may form a
discontinuous phase). It is also preferred that the polyester
polymers do not contain any fillers, fibers, or impact modifiers or
other polymers which form a discontinuous phase.
The polyester compositions can be prepared by polymerization
procedures known in the art sufficient to effect esterification and
polycondensation. Polyester melt phase manufacturing processes
include direct condensation of a dicarboxylic acid with the diol,
optionally in the presence of esterification catalysts, in the
esterification zone, followed by polycondensation in the prepolymer
and finishing zones in the presence of a polycondensation catalyst;
or ester exchange usually in the presence of a transesterification
catalyst in the ester exchange zone, followed by prepolymerization
and finishing in the presence of a polycondensation catalyst, and
each may optionally be solid stated according to known methods.
Once the polyester polymer is manufactured in the melt phase
polymerization, it is solidified. The method for solidifying the
polyester polymer from the melt phase process is not limited. For
example, molten polyester polymer from the melt phase may be
directed through a die, or merely cut, or both directed through a
die followed by cutting the molten polymer. A gear pump may be used
as the motive force to drive the molten polyester polymer through
the die. Instead of using a gear pump, the molten polyester polymer
may be fed into a single or twin screw extruder and extruded
through a die, optionally at a temperature of 190.degree. C. or
more at the extruder nozzle. Once through the die, the polyester
polymer can be drawn into strands, contacted with a cool fluid, and
cut into pellets, or the polymer can be pelletized at the die head,
optionally underwater. The polyester polymer melt is optionally
filtered to remove particulates over a designated size before being
cut. Any conventional hot pelletization or dicing method and
apparatus can be used, including but not limited to dicing, strand
pelletizing and strand (forced conveyance) pelletizing,
pastillators, water ring pelletizers, hot face pelletizers,
underwater pelletizers and centrifuged pelletizers.
The polyester polymer may also be crystallized if desired as noted
above. The method and apparatus used to crystallize the polyester
polymer is not limited, and includes thermal crystallization in a
gas or liquid. The crystallization may occur in a mechanically
agitated vessel; a fluidized bed; a bed agitated by fluid movement;
an un-agitated vessel or pipe; crystallized in a liquid medium
above the T.sub.g of the polyester polymer, preferably at
140.degree. C. to 190.degree. C.; or any other means known in the
art. Also, the polymer may be strain crystallized. The polymer may
also be fed to a crystallizer at a polymer temperature below its
T.sub.g (from the glass), or it may be fed to a crystallizer at a
polymer temperature above its T.sub.g. For example, molten polymer
from the melt phase polymerization reactor may be fed through a die
plate and cut underwater, and then immediately fed to an underwater
thermal crystallization reactor where the polymer is crystallized
underwater. Alternatively, the molten polymer may be cut, allowed
to cool to below its T.sub.g, and then fed to an underwater thermal
crystallization apparatus or any other suitable crystallization
apparatus. Or, the molten polymer may be cut in any conventional
manner, allowed to cool to below its T.sub.g, optionally stored,
and then crystallized.
In each of these embodiments, the articles of manufacture are not
limited, and include sheet and bottle preforms. The bottle preforms
can be stretch blow molded into bottles by conventional processes.
Thus, there is also provided in an embodiment the bottles made from
the particles of the invention, or made by any of the processes of
the invention, or made by any conventional melt processing
technique using the particles of the invention.
Not only may containers be made from particles made according to
the process of this invention, but other items such as sheet, film,
bottles, trays, other packaging, rods, tubes, lids, filaments and
fibers, and other molded articles may also be manufactured using
the polyester particles of the invention. Beverage bottles made
from polyethylene terephthalate suitable for holding water or
carbonated beverages, and heat set beverage bottle suitable for
holding beverages which are hot filled into the bottle are examples
of the types of bottles which are made from the crystallized
pellets of the invention.
FIGS. 1 and 2 illustrate non-limiting process flow embodiments
describing how the invention could be practiced.
In FIG. 1, a stream of hot crystalline polyester particles
containing a level of residual AA greater than 10 ppm is introduced
into a vessel 105 through particle inlet pipe 101. The particles
form a bed 106 within the vessel 105 and move downward toward the
vessel outlet 103 to form a stream of crystalline polyester
particles having a reduced level of residual acetaldehyde of 10 ppm
or less. A stream of gas is fed into the vessel through a side
inlet 103 toward the lower 1/3 of the vessel height. Other suitable
locations, not illustrated, include a bottom inlet closer toward
the particle outlet 103, or a top feed. After picking up
acetaldehyde from the particles, the gas is removed from the vessel
105 through gas outlet 104. The location of 103 and 104 relative to
each other are preferably chosen so that gas flows across the
majority of the particles in the bed 106.
The polyester particle stream flowing into particle inlet 101 are
at a temperature of 120 to 180.degree. C., and contain more than 10
ppm acetaldehyde. The stream rate of the particles is not limited
as this process will be effective at a very wide range of rates.
The mass of the particles in the bed 106 is selected to give the
desired residence time for particles in the vessel 105. For
example, if the rate of particles in stream 101 is 10,000 lbs/hr,
and an average residence time of 20 hours is desired, the mass of
particles in the bed 106 should be (10,000 lb/hr)(20 hr)=200,000
lbs. The size of the vessel 105 is sufficient to contain the bed
106.
Preferably the vessel 105 is insulated to prevent unnecessary heat
losses. The average temperature of the particles in the bed 106 is
within 120.degree. C. and 180.degree. C. and will depend primarily
on the temperature, rate, and feed location of particle stream
though particle inlet 101, the temperature, rate, and feed location
of gas through inlet 102, and heat losses from the vessel 105. At
low inlet gas rates, the gas stream will not have a large impact on
the average temperature of particles in the bed 106.
Particles are removed 106 from the vessel containing less than 10
ppm acetaldehyde. The temperature of stream 103 is not limited and
depends primarily on the temperature and rate of incoming particles
101 the temperature and rate of inlet gas 102, and heat losses from
the vessel 105. The level of acetaldehyde in stream 103 depends
primarily on the rate and acetaldehyde content of particles in
inlet stream 101, the temperature and mass of particles in bed 106,
the rate and temperature of gas 102, fed to the vessel, and the
rate at which acetaldehyde is chemically generated in the polymer
during the stripping process. At steady state, the rate of pellet
removal 106 is on average the same as the rate of particles at the
inlet 101. One skilled in the art is aware that these rates may be
intentionally set differently to adjust the mass of the bed
106.
The rate at the gas inlet 102 is preferably greater than 0.0001
SCFM per lb/hr of particles 101 fed to the stripper. There is a
balance between having sufficient gas to dilute the acetaldehyde
and ensure a large driving force for acetaldehyde to leave the
polymer particles, versus the cost of providing higher gas rates to
the stripper. At the low gas rates that are preferred, the
temperature of the gas is not limited as it does not have a large
impact on the temperature of particles in the bed 106. At high gas
flow rates, for example 1 lb/hr of air in stream 102 per 1 lb/hr of
particles in stream 101 the gas temperature can have a significant
impact on the temperature of bed 106 and must be chosen to give a
bed temperature between 120 and 180.degree. C. The inlet gas stream
102, is preferably air substantially free of acetaldehyde.
The rate at the gas outlet 104 is on average the same as the
average rate of the gas inlet 102. The temperature is not limited,
and will depend primarily on the temperature of the bed 106 through
which the gas has last flowed before exiting the vessel. The
concentration of acetaldehyde at the gas outlet 104, will depend on
the amount of acetaldehyde removed from the polymer particles and
the gas flow rate.
FIG. 2 is another non-limiting example of an embodiment in which
the heat energy from the particles imparted during crystallization
is integrated with the energy required for stripping AA. As
illustrated in FIG. 2, a molten polyester polymer stream is fed to
an underfluid cutter 203 through line 201 using a gear pump 202 as
the motive force. While an underfluid cutter is illustrated, any
conventional type of pelletizer can be employed to make pellets
which are eventually fed to a crystallizer. The source of the
molten polymer may be from pellets fed through an extruder to the
gear pump 202 or from the melt phase high polymerizer or finisher
(not shown) fed to the gear pump 202. The liquid medium is fed into
cutter 203 through a feed pipe 206 into the cutter 203. A suitable
liquid medium comprises water entering the housing at a fluid
velocity of 1 ft/s to 8 ft/s, preferably 1 ft/s to 4 ft/s. The flow
of liquid medium through the cutter 203 sweeps the cut particles
away from the cutter and into the outlet pipe 208 for transport
into a crystallizer 209.
As illustrated, the crystallizer 209 is an underfluid crystallizer
having a high liquid temperature in which the liquid is kept under
a pressure equal to or greater than the vapor pressure of the
liquid to keep the fluid in the liquid state. Crystallizer 203
comprises of a series of pipes in a coil or stacked to form a three
dimensional box or any other shape, including a long linear tube.
The liquid (e.g. water) temperature at the outlet pipe 208 and
through the crystallizer pipes 209 is above the T.sub.g of the
particles, and preferably at any temperature within a range of
greater than 100.degree. C. to 190.degree. C., and more preferably
from 140 to 180.degree. C. While underfluid crystallizer is
illustrated, any conventional crystallizer is suitable. For
example, a suitable crystallization method includes passing a
countercurrent gas of hot nitrogen or air or both at a gas feed
temperature of 160.degree. C. to 220.degree. C. through a bed of
solid pellets agitated by the gas flow or by mechanical agitation,
or alternatively, the heat source to the pellets is provided by
heat transfer through the jacketed walls of a vessel. The particles
attain a degree of crystallization ranging from 20% to about 65%,
or about 25% to about 50% after discharge from the
crystallizers.
After flowing through the crystallization pipes, the crystallized
particles are fed through pipe 210 to a particle/liquid separator
211. A separator 211 is not needed, however, if conventional
crystallization techniques are applied which use a gas or the walls
of a vessel as the heat transfer source. The method or equipment
for separating particles from liquid is not limited. Examples of
suitable separators include centrifugal dryers, solid or screen
bowl centrifuges, pusher centrifuges, or simple filters or screens
into which the particle/liquid slurry is fed with the liquid
flowing through the screen and out through liquid outlet pipe 212.
The liquid in pipe 212 may optionally be re-circulated as a source
of liquid for the feed into the underfluid cutter.
The particles are discharged from separator 211 through particle
outlet pipe 213 and fed into vessel 105, the AA stripping vessel.
In the event that a conventional crystallizer is used, the
particles can be fed directly or indirectly from the crystallizer
to the AA stripping vessel 105. The particles fed to the vessel 105
have high heat energy imparted by the crystallizer 209. The heat
energy in the particles is used as the source of heat transferred
to the gas supplied to the vessel 105 through line 103 which flows
through the particle bed 106.
In this embodiment, the polyester particle stream is fed into
vessel 105 at a temperature of at least 50.degree. C. The
crystallized particle stream discharged from the separator 211, or
discharged from a conventional crystallizer, is typically at a
temperature in excess of 90.degree. C., or in excess of 120.degree.
C., or in excess of 130.degree. C. Between the conventional
crystallizer, or the separator 211, and the stripping vessel 105,
the particles may cool somewhat through heat losses to the piping,
or heat losses in the separator 211, or within optional equipment
between the separator 211 and the vessel 105. Between the discharge
from the crystallizer, whether conventional or as illustrated in
FIG. 2 as 109, the temperature of the crystallized particles
preferably does not drop below 50.degree. C., or does not drop
below 75.degree. C., or does not drop below 90.degree. C., or does
not drop below 100.degree. C., or does not drop below 110.degree.
C. In this embodiment, the stream of crystallized particles is fed
into the stripping vessel 105 through particle inlet pipe 101 at a
temperature of at least 130.degree. C., while a flow of gas is fed
through gas inlet 102 and through the bed of crystallized particles
106. The feed temperature of at least 130.degree. C. is preferred
because at lower temperatures, the residence time of the particles
in the vessel is undesirably long. Finished particles are
discharged through particle outlet line 103 and the gas is
discharged preferably toward the top of the vessel 105 through a
gas discharge line 104.
In the event that the temperature of the crystallized particles
from a crystallizer or from a liquid/solid separator drops below
130.degree. C., the stream of crystallized particles can be
reheated to at least 130.degree. C. by any conventional heating
means. Even though thermal energy may be to be applied to reheat
the stream of crystallized particles, the integrated process
requires the application of less energy than would be required if,
for example, the particle temperature falls to ambient temperature.
Suitable heating devices include pre-heaters or thermal screws.
Experiment Set 1
This set of experiments illustrates the effects of time and
temperature on the residual acetaldehyde, molecular weight, color,
and crystallinity of the polyester polymer particles.
Three different polyethylene terephthalate based polymers
representing three different geometries were placed in a fluidized
bed reactor and exposed to either 150.degree. C., 160.degree. C.,
or 185.degree. C. temperatures and a low air flow rate for at least
24 hours. More specifically, the experiments were conducted in a
column reactor comprised of a modified chromatography column to
allow for the introduction of a gas stream over the polymer
particles and to regulate the temperature of the polymer particles,
a round bottom flask, and a condensor.
The column reactor is illustrated in FIG. 3. The outside glass wall
301 contains an inside glass wall 302 within which is a chamber 303
for polymer particles. At the bottom of the chamber 303 is a
fritted glass support 304, through which is fed a gas at a gas
inlet port 306 flowing through a coil of glass tubing 305. On the
outside glass wall is provided a connector 307 for a round bottom
flask and a connector 308 for a condenser.
The temperature of the column reactor, polymer particles within the
column and the gas flowing over the polymer particles in the column
is regulated by refluxing a suitable solvent in a round bottom
flask connected to the column at inlet 307. A condenser is attached
to the column at 8 to allow for the refluxing solvent to be
reclaimed to the reactor. Cumene (b.p.=150.degree. C.),
cyclohexanol (b.p.=160.degree. C.) or diethyl octoate
(b.p.=185.degree. C.) was used as the temperature regulating
solvent.
The experiments were conducted in two stages by charging the vessel
with 1.5 pounds (680 g) of a partially crystallized PET resin. In
the first set of experiments, the resin was charged to the vessel
at 7:00 a.m., and about 60 grams samples were collected at each
time interval indicated on Table 1. In a second set of experiments,
the resin was charged to the vessel at 5:00 p.m., and about 60
grams samples were collected at each time interval as indicated on
Table 1 below. The samples were submitted for residual acetaldehyde
analysis using the test method as described above, for inherent
viscosity test measurements as described above, to color
(reflectance) analysis as described above, and for % crystallinity
analysis as described above.
Within each set of experiments, three different runs were made. In
the first run, a polyester polymer thermally crystallized at
175.degree. C. to a degree of crystallinity of 33% and having an
It.V. of 0.816 was used ("Polymer 1"). In the second run, a
polyester polymer crystallized with a roll processing unit to a
degree of crystallinity of 35.7% and having an It.V. of 0.802 was
used ("Polymer 2"). In the third run, a polyester polymer
crystallized underwater to a degree of crystallinity of 30.5% and
having an It.V. of 0.820 was used ("Polymer 3"). In each case, the
polyester polymer was a polyethylene terephthalate based polymer
having 2.0 mol % (of total dicarboxylic acid content) isophthalic
acid modification. The average particle dimensions were about
1.84.times.2.68.times.2.43 mm, 2.45.times.3.09.times.3.90 mm, and
2.75 mm diameter, respectively.
Within the second set of experiments, one run was performed using
Polymer 1, except the second experiment was performed at the higher
temperature of 160.degree. C.
Within the third set of experiments, three runs were performed
using the same polymers as in the first set of experiments, except
that the third set of experiments was performed at the higher
temperature of 185.degree. C.
The air flow for each experiment was set at 0.0067 SCFM using
ambient plant air. The amount of solvent charged to the round
bottom flask connected to the column reactor was 1000 ml. The
residence time of the particles was varied and are detailed in
Tables 1 through 7 in each case. The polymer charge was 1.5 lbs in
each case. The polymer was added to the column reactor after the
column had reached the target temperature of 150.degree. C.,
160.degree. C., or 185.degree. C., depending upon the solvent used
in each set of experiments. The time at which the polymer was added
to the vessel was set as the start time for the experiment (Time=0
hr). The temperature of the polymer particles was measured by a
thermocouple placed on the fritted glass support (4 in FIG. 3) The
results of the experiments run at 150.degree. C. using Polymer 1
are reported on Table 1. The results of the experiments run at
160.degree. C. using Polymer 1 are reported on Table 2. The results
of the experiments run at 185.degree. C. using Polymer 1 are
reported on Table 3. The results of the experiments run at
150.degree. C. using Polymer 2 are reported on Table 4. The results
of the experiments run at 185.degree. C. using Polymer 2 are
reported on Table 5. The results of the experiments run at
150.degree. C. using Polymer 3 are reported on Table 6. The results
of the experiments run at 185.degree. C. using Polymer 3 are
reported on Table 7.
TABLE-US-00001 TABLE 1 Polymer 1, 150.degree. C., 0.0067 SCFM
Residual Elapsed Acetaldehyde It.V. % Time (hr) (ppm) (dl/g) L* a*
b* Crystallinity 0.00 45.23 0.811 65.78 -1.345 -3.06 32.6 1.00
34.91 0.803 66.1 -1.33 -3.20 34.2 2.00 32.28 0.815 66.33 -1.32
-3.29 32.3 3.00 25.43 0.812 66.48 -1.32 -3.38 33.9 3.83 19.98 0.810
66.44 -1.23 -3.37 31.9 6.33 10.95 0.812 66.41 -1.27 -3.23 31.3 8.00
7.26 0.821 66.76 -1.22 -3.28 34.5 9.50 6.00 0.819 66.90 -1.25 -3.25
38.3 14.00 5.37 0.803 66.69 -1.18 -3.29 33.5 16.00 3.39 0.813 67.37
-1.19 -3.41 30.6 18.00 2.95 0.816 66.35 -1.16 -3.17 32.5 20.50 2.63
0.816 67.35 -1.15 -3.36 32.4 22.58 2.54 0.816 67.29 -1.18 -3.46
34.8 23.50 2.50 0.821 67.10 -1.15 -3.43 35.7 23.75 2.45 0.829 66.86
-1.1933 -3.21 33.7
TABLE-US-00002 TABLE 2 Polymer 1 at 160.degree. C., 0.0067 SCFM
Residual Elapsed Acetaldehyde It.V. % Time (hr) (ppm) (dl/g) L* a*
b* Crystallinity 0.00 57.02 0.831 65.29 -1.37 -3.37 28.42 1.50
47.40 0.831 66.08 -1.31 -3.70 26.55 2.50 28.95 0.823 66.85 -1.28
-3.78 28.52 3.50 20.55 0.821 66.38 -1.21 -3.60 27.57 4.83 12.51
0.813 66.48 -1.18 -3.48 27.44 6.75 7.16 0.822 67.07 -1.20 -3.73
28.99 8.58 5.22 0.822 65.99 -1.09 -3.41 30.21 23.75 3.00 0.810
66.50 -1.06 -3.40 30.06
TABLE-US-00003 TABLE 3 Polymer 1 at 185.degree. C. and 0.0067 SCFM
Residual Elapsed Acetaldehyde It.V. Time (hr) (ppm) (dl/g) L* a* b*
% Crystallinity 0.00 43.36 0.821 65.98 -1.35 -3.12 34.24 1.00 34.90
0.810 65.49 -1.30 -3.12 33.15 2.00 13.78 0.809 67.43 -1.20 -3.36
32.95 3.00 8.13 0.818 67.69 -1.13 -3.21 31.78 4.00 6.85 0.820 66.94
-1.07 -3.02 30.77 5.50 6.44 0.808 67.52 -0.99 -2.90 37.23 7.50 5.13
0.826 67.16 -0.96 -2.36 36.85 15.00 2.92 0.813 68.68 -0.73 -2.05
38.99 17.17 2.40 0.827 68.82 -0.71 -1.77 40.49 18.50 2.21 0.845
68.46 -0.66 -1.69 38.36 20.00 1.78 0.858 69.36 -0.63 -1.68 39.68
21.33 1.71 0.857 69.47 -0.67 -1.53 38.05 23.00 1.48 0.852 68.56
-0.55 -1.28 40.64 23.17 1.25 0.826 69.47 -0.6 -1.34 38.53
TABLE-US-00004 TABLE 4 Polymer 2 at 150.degree. C. and 0.0067 SCFM
Residual Elapsed Acetaldehyde It.V. Time (hr) (ppm) (dl/g) L* a* b*
% Crystallinity 0.00 7.52 0.800 55.43 -0.97 -0.06 36.18 2.00 6.08
0.809 55.84 -1.00 -0.23 40.91 3.00 5.11 0.807 56.03 -1.07 -0.21
34.31 4.42 4.19 0.810 56.42 -1.02 -0.31 41.44 6.00 3.47 0.806 56.02
-1.04 -0.31 45.63 7.58 2.94 0.812 56.47 -1.03 -0.56 40.89 9.42 2.53
0.797 56.91 -0.95 -0.45 41.97 14.00 1.71 0.793 56.59 -0.94 -0.18
36.38 16.00 1.61 0.804 55.16 -0.95 -0.48 52.75 18.00 1.38 0.801
56.5 -0.98 -0.47 42.43 20.08 1.24 0.803 56.32 -0.97 -0.41 37.04
22.00 1.22 0.797 56.43 -0.95 -0.48 41.59 23.92 1.14 0.800 57.17
-0.98 -0.58 42.00 24.50 1.04 0.804 56.50 -0.95 -0.45 37.18 39.25
0.86 0.797 56.35 -0.94 -0.47 47.70
TABLE-US-00005 TABLE 5 Polymer 2 at 185.degree. C. and 0.0067 SCFM
Residual Elapsed Acetaldehyde It.V. Time (hr) (ppm) (dl/g) L* a* b*
% Crystallinity 0.00 20.67 0.810 55.49 -0.80 -0.32 35.20 1.00 5.415
0.799 56.53 -0.85 -0.59 35.67 1.83 3.63 0.786 56.00 -0.78 -0.46
35.70 2.83 2.60 0.812 56.70 -0.81 -0.6 35.69 4.75 1.72 0.793 56.91
-0.78 -0.72 45.51 6.83 1.18 0.802 55.58 -0.77 -0.38 40.25 12.75
0.84 0.798 57.65 -0.78 -1.04 40.15 14.50 0.79 0.797 57.36 -0.78
-0.72 37.84 16.42 0.69 0.803 57.91 -0.81 -0.61 40.64 18.25 0.63
0.816 57.85 -0.75 -0.77 41.98 21.00 0.65 0.815 57.88 -0.77 -1.00
41.06
TABLE-US-00006 TABLE 6 Polymer 3 at 150.degree. C. and 0.0067 SCFM
Residual Elapsed Acetaldehyde It.V. Time (hr) (ppm) (dl/g) L* a* b*
% Crystallinity 0.00 19.83 0.800 68.21 -1.78 -2.17 33.74 1.00 16.94
0.794 68.39 -1.74 -2.20 37.90 2.08 12.38 0.806 69.12 -1.71 -2.20
33.55 3.00 9.61 0.807 68.98 -1.74 -2.17 33.93 4.00 7.37 0.744 69.21
-1.68 -2.13 33.15 5.08 6.41 0.854 69.23 -1.65 -2.20 34.67 6.08 4.72
0.848 69.26 -1.71 -1.99 34.29 8.00 3.26 0.791 69.02 -1.68 -2.09
31.14 13.67 1.44 0.796 69.65 -1.64 -2.20 39.72 16.00 1.17 0.809
69.83 -1.65 -2.27 44.10 18.00 1.02 0.840 69.45 -1.65 -2.21 37.24
20.00 0.91 0.835 69.59 -1.65 -2.15 38.29 21.67 0.84 0.792 69.83
-1.62 -2.13 31.35 22.00 0.81 0.840 69.69 -1.64 -2.10 46.21 24.00
0.79 0.791 69.76 -1.64 -2.15 39.03
TABLE-US-00007 TABLE 7 Polymer 3 at 185.degree. C. and 0.0067 SCFM
Residual Elapsed Acetaldehyde It.V. Time (hr) (ppm) (dl/g) L* a* b*
% Crystallinity 0.00 18.01 0.840 67.86 -1.61 -2.45 27.28 1.00 13.81
0.831 68.95 -1.63 -2.37 29.90 2.00 4.76 0.825 70.29 -1.50 -2.21
29.10 3.00 2.09 0.813 71.15 -1.46 -2.14 31.60 5.00 1.51 0.830 71.45
-1.45 -1.96 27.64 7.00 1.27 0.836 71.60 -1.43 -1.80 34.25 10.00
1.05 0.844 71.12 -1.40 -1.81 34.23 14.00 0.81 0.849 71.87 -1.38
-1.64 35.58 18.00 0.57 0.859 71.98 -1.38 -1.49 36.54 23.00 0.38
0.880 71.95 -1.35 -1.37 37.30
The results indicated that for all temperatures tested, 150.degree.
C., 160.degree. C., and 185.degree. C., the level of residual
acetaldehyde remaining after 24 hours was less than 3 ppm for all
samples tested. When the process was conducted at 185.degree. C.,
an increase in molecular weight was observed due to the
polycondensation reactions occurring at this high temperature.
Also, at 185.degree. C., a significant increase in L* was observed,
and an increase in the a* and b* color value were also observed.
However, when the process temperature was lowered to below
160.degree. C., no significant change in the molecular weight, L*,
a* or b* was observed. Based upon the experimental observations,
one may conclude that residual acetaldehyde formed during the melt
phase polymerization of PET may be effectively removed by exposing
the resin to a flow of gas at a temperature which does not
significantly affect the fitness of the particles for its desired
use as indicated by insubstantial changes in the It.V., L*, or b*
color values of the particles. The finding that the b* color value
can remain unchanged in the presence of atmospheric oxygen is an
important consideration because in solid state polymerization
operations, great care is taken to minimize the concentration of
oxygen to prevent changes in b* color at the high temperature
conditions.
* * * * *